1. Field of the Invention
The invention relates generally to a capacitor and a method of manufacturing the same. More particularly, the disclosed to a capacitor and a method of manufacturing the same, which can improve the step coverage of a BST dielectric film formed by a MOCVD method and that can prevent a change in the composition ratio to improve the dielectric characteristic of a capacitor.
2. Description of the Prior Art
The use of a high dielectric BST thin film as a capacitor material for Giga DRAMs has been studied. In a device having the cell size of less then 0.10 μm2, even though the BST thin film is used as a dielectric material, a lower electrode must be formed in a three-dimensional structure or a capacitor must be formed with a concave structure. At this time, if the structure is etched using a noble metal such as Pt, Ru, Ir, etc. in order to form a lower electrode, the upper portion of the lower electrode is not etched with a vertical profile of 90°. It is at most etched with a non-vertical profile of about 80°. Therefore, it is impossible to use this method in a cell of that is smaller than 0.10 μm2.
As a result, in order to solve this problem, a capacitor with a concave structure must be manufactured. At this time, in order to form such an electrode and a dielectric film, it is necessary that the CVD method have good step coverage. However, the types of precursors for the Pt CVD being developed now have been limited and they are also expensive. Therefore, it is economical to use a Ru precursor due to its lower cost. The capacitor manufactured thus is mainly used to store data in a DRAM.
FIG. 1 is a cross-sectional view for explaining a method of manufacturing a conventional capacitor. A first insulating film 2 is formed on a semiconductor substrate 1 in which various components for forming a semiconductor device are formed. Then, a contact hole is formed so that a given junction can be exposed. Next, a polysilicon layer for contact plug 3 is formed within the contact hole and is then flatten by chemical mechanical polish process, thus forming a Si3N4 film 6 and a second insulating film 7 on the entire structure.
Thereafter, a given region of the second an insulating film 7 is etched to form an opening wider than the contact hole, while exposing the polysilicon plug 3. Then, barrier layers 4 and 5, and a Ru lower electrode 8 are formed at the sidewall and at the bottom of the opening. Next, a BST dielectric film 9 and an upper electrode 10 are formed on the entire structure thereby covering the Ru lower electrode 8.
Conventionally, with a method of manufacturing a capacitor having a concave structure, the area of the lower electrode occupies less than 10% of the total wafer area. In other words, seeing the surface of the wafer before a MOCVD BST dielectric film is formed, most of the surface is a SiO2 film being an insulating layer and the lower electrode occupies a very small area.
However, the BST dielectric film is made of 4-element series such as Ba, St, Ti and O2, and if it is formed by CVD method, the composition ratio of a thin film is different depending on the condition of an underlying substrate. Also, in a capacitor of a concave structure or a stacked structure, as the cell size of the BST dielectric film is reduced even though it is deposited by a CVD method, the aspect ratio becomes relatively larger, thus significantly degrading the step coverage characteristic.
In a conventional concave structure, the step coverage after the MOCVD BST dielectric film is deposited is at most about 50%, which significantly makes it difficult to manufacture a capacitor structure in a capacitor having the cell size of 0.1 μm2. That is, in case of a concave structure, as BST of a vapor state, made of 4-element series such as Ba, St, Ti and O2 enters holes to form a BST dielectric film, a poor step coverage results in comparison to TiN or other thin films. Even in the case that the lower electrode has a stack structure, as most of the wafer is a SiO2 film and an electrode occupies a very small area, the step coverage due to the difference of temperature is poor. Further, in case that the composition ratio of the BST dielectric film is different, it may have a critical affect on the electrical characteristics of a capacitor.
However, in the case that a lower electrode of a conventional capacitor is formed, if a MOCVD BST dielectric film is to be formed, the composition ratio of a BST dielectric film deposited on a SiO2 and a BST dielectric film deposited on a lower electrode will be different. Also, as the composition ration of the BST dielectric film deposited on the lower electrode is different from that expected, a capacitor having a good electrical property cannot be obtained. Thus, in order to form a BST dielectric film having a desired composition ratio, the process temperature must be increased. In that case, a TiN/Ti film used as a diffusion barrier layer is oxidized, and particles are inevitably generated during the process of forming a BST dielectric film.